1. Field of Invention
This invention relates in general to the mounting of microwave devices on circuit boards.
2. Description of Related Art
In power amplifiers operating at frequencies below 30 MHz, the chassis on which the amplifier is mounted effectively operates as a fixed ground reference, since its dimensions are only a fraction of a wavelength. During operation, an object affixed to the chassis may be assumed to be at ground potential. At frequencies above 30 MHz, however, the chassis dimensions and characteristics become significant and it must be considered to be the electrical equivalent of an inductor or transmission line.
In amplifier stages, it is important to have the shortest possible path between the amplifier ground and the ground of the transmission line it interfaces with. In large volume production this ground path must also be very consistent and guaranteed for proper function and reliability.
An amplifier is often mounted directly to the chassis for best heat sinking. If the amplifier's input and/or output leads connect to a circuit board transmission line, the chassis must provide the ground path between the amplifier ground and the circuit board transmission line ground. Small imperfections in the surface of the chassis, amplifier, or circuit board may create a much longer ground path and create a discontinuity between the amplifier and the transmission line. A discontinuity in a transmission line causes undesirable reflections of radio frequency signals that can adversely effect the amplifier's performance.
Surface imperfections may reduce the contact area between the amplifier and the chassis and result in reduced heat sinking for the amplifier. In order to assure the best possible contact between the transistor and chassis, a very flat and smooth surface finish is typically machined into the chassis at a location where the transistor seats. Superior flatness and surface finish provides greatest contact between mating components, which in turn, provides increased thermal and electrical conductivity between components. However, as in most machining operations where different components are manufactured or machined on different machine set-ups, post-machining is typically required of at least one component in order for its mating surface to confonn to its counterpart within the required specifications, or to merely correct imperfections in the initially machined step. Thus, prior to assembly, the location on the chassis where the transistor is seated is typically post-machined in order to meet the flatness and/or surface finish requirement of the interface. To facilitate connection to ground, a thin piece of aluminum is typically placed between the transistor, or other chassis-mounted component, which tends to conform to surface variations in both the transistor and chassis. Post-machining prior to assembly, however, is still required, and though effective in producing an adequate finish on the chassis, this practice is expensive and time consuming.